1. Field of the Invention
This invention relates to magnetic heads. More particularly, the invention is a structure for a read-after-write magnetic head having improved means for reducing flux leakage between circuits.
2. Description of the Related Art
The principle of placing information upon a moving magnetizable surface, and subsequently deriving the information therefrom, has been utilized for many years in various types of apparatus. Perhaps the most common use is the computer, which makes use of magnetizable storage media such as magnetic tape for storing and recording information in digital form. Information is stored on small areas of the tape surface, known as magnetic domains. The magnetic domains are microscopic in size and can be oriented to form a magnetized area of one of two polarities, representing digital zeros or ones. The magnetic domains are oriented by placing the tape in a magnetic field. The tape is a plastic ribbon coated with a "hard" magnetic material, i.e. one which retains its magnetization after it has passed through a magnetic field. Later, the information on the tape can be sensed magnetically. By appropriately combining the digital signals in a known manner, large amounts of information can be stored on the magnetic tape. The information may be recorded in any one of several binary data codes.
Read-after-write magnetic heads are used for the writing and reading of information in computer peripherals such as magnetic tape drives. These heads may have one or more tracks, with each track having closely spaced, parallel read and write core gaps. Each core gap is formed by a pair of core pieces, hereinafter referred to together as a magnetic core, with a coil mounted on one core piece. A "gap" may extend beyond the core for manufacturing purposes, but only that portion formed by the core pieces is referred to as the "core gap". The core gap, magnetic core, and coil are hereinafter referred to as a magnetic flux circuit, a magnetic transducer, or simply a magnetic circuit. The input to the write coil is a digital electrical signal comprising a series of pulses. The excitation current from the signal produces magnetic field lines that diverge from the gap and penetrate magnetic tape moving past the write gap from a tape supply reel. In this manner, information in the form of digital zeros and ones is recorded on the magnetic tape. The magnetic heads have "soft" magnetic cores; magnetization is not retained therein after the excitation current is removed.
Immediately following recording, the magnetic tape passes over the read gap where the recorded information is sensed to check the accuracy of the recording. The read and write functions are carried on simultaneously by the magnetic head so that the accuracy of the written information can be verified without interruption of the writing process. As the magnetic tape continues past the write gap and passes over the read gap, magnetic field lines from the recorded tape permeate the core of the read gap and produce an induced voltage in the read coil. From this induced voltage, the information recorded on the tape can be reproduced.
Because of the extremely close proximity of the read and write magnetic flux circuits, and because of the relatively high current flow through the write coil, the write signal tends to be fed over and sensed by the read circuit. This coupling of the read and write circuits is known as crossfeed or feedthrough. If the feedthrough caused by leakage flux is too high, the signal induced in the read circuit by the leakage flux from the write circuit may be so large as to mask the signal sensed from the tape. Thus, feedthrough can prevent one from properly utilizing the magnetic tape by rendering the output of the read coil meaningless. This problem has heretofore been dealt with in a number of ways.
One method of reducing feedthrough includes the use of copper shield elements inlaid in the surface of the magnetic core of a magnetic head. "Low Noise Magnetic Recording Head", IBM Technical Disclosure Bulletin, Vol. 8, No. 4, Sept., 1965, pp. 499-500. The copper shield elements provide two benefits. First, feedthrough is reduced because the portions of the core pieces that are exposed are significantly reduced. Also, the magnetic field that is emitted across the gap of the head is increased because the fields generated within the head are forced out through the small exposed portions and gap of the magnetic core. However, the increased field emitted from the head to the tape can actually increase the risk of feedthrough along the path from a write gap to the tape and into a read gap. In addition, metallic inlays tend to create magnetic eddy currents which disrupt the magnetic performance of a head.
Other shielding schemes exist which use shields manufactured at least in part from soft permeable magnetic materials. U.S. Pat. No. 3,042,753 discloses a magnetic shield which is a casing almost completely encapsulating a discrete magnetic core. The shield is used to protect the core from stray magnetic fields. This scheme suffers in that each discrete magnetic core is individually encapsulated by a distinct shield. In modern magnetic devices in which magnetic heads are built with several gaps integrated into a single unit, it is impractical to separately shield each magnetic gap in such a manner.
U.S. Pat. No. 3,744,040 discloses a shield which is specifically used to prevent feedthrough. This shield is C-shaped such that it nearly surrounds the entire magnetic core. This shield suffers in that the encapsulating shape and the extreme depth of the shield with respect to that of the core means that the shield will be expensive to produce. The use of such a large shield requires much manufacturing time and materials. In addition, shields such as this which are relatively thin on the surface of the magnetic head, do not provide a effective feedthrough prevention mechanism for the feedthrough path extending from the write gap along the tape path and into the read gap. Finally, a merely deep shield may not adequately prevent feedthrough in heads having raised read and write gaps because an unshielded area may exist therebetween.
Another feedthrough shield is disclosed in U.S. Pat. No. 3,806,902. This patent discloses a set of shields which are interconnected to surround, or nearly surround, each magnetic core. However, while the shield pieces are shallow in certain areas, they tend to be quite deep in the region located between the read and write portions of the head. Thus, this shielding technique suffers from the same problems discussed with respect to U.S. Pat. 3,744,040.
A problem similar to feedthrough in magnetic heads is known as cross-talk. Cross-talk is the interference not between the read and write components of a magnetic head, but between two read components or two write components across different tracks of a head. Jorgensen discloses that cross-talk can be greatly reduced by the insertion of deep shields between the cores. Jorgensen, Finn, "The Complete Handbook of Magnetic Recording", July, 1980, pp. 152-154. It is further disclosed that full shields must be used because any split in the shields may create a secondary recording gap and will reduce the shielding efficiency. Thus, Jorgensen teaches that the shields used to reduce cross-talk must be both deep and unbroken. Heads manufactured accordingly would again suffer from the aforementioned problems.